The OTDM technique is particularly interesting in all situations where the need arises to increase the transmission capacity of an optical link and is an alternative to other solutions based on sharing the same physical carrier among multiple channels, typically by employing WDM (Wavelength Division Multiplexing) techniques, or on increasing the number of optical fibers available for the link.
This latter solution generally requires intervention on installation (such as laying of new cables and performing excavation) and does not completely exploit the extremely wide band made available by optical fibers.
The simultaneous transmission of many different channels on the same optical fiber, for example according to wavelength division multiplexing techniques, allows using low speed opto-electronic components both in the transmitter and in the receiver, while obtaining a high overall capacity on the link. Wavelength division multiplexing further allows implementing at the optical level of some network functions like channel removal and insertion, dynamic routing, link protection, with a high reduction of the processing load for the electronic part in network nodes. The major inconveniences of such method are linked to the need for selecting and stabilizing the wavelengths for transmitters and optical filters used for channels selection, to the possible inter-channel interference due to non-linear phenomena in fiber propagation (for example the phenomenon known as Four Wave Mixing) or to the spectral nonuniformity of optical amplifier gain.
In OTDM systems, many optical signals, intensity modulated according to an RZ (return to zero) code, are interleaved into a single flow by acting on the relative delay of the pulse sequences. This solution retains most of the advantages of WDM techniques related to the possible use of low speed opto-electronic components both in the transmitter and in the receiver, further avoiding the onset of some of the above-mentioned negative phenomena. A basic condition for the proper operation of an OTDM system is however that the different optical tributary flows must be well synchronized and composed of sufficiently narrow pulses in order to avoid interference among channels. Moreover, it is essential that a driving signal at tributary frequency and synchronous with the multiplexed flow is available at the demultiplexing device.